Essentials of Anatomy and Physiology

digestive secretions decrease and peristalsis slows;
these are not important in a stress situation.
Vasoconstriction in the skin and viscera shunts blood
to more vital organs such as the heart, muscles, and
brain. All of these responses enabled our ancestors to
stay and fight or to get away from potential danger.
Even though we may not always be in life-threatening
situations during stress (such as figuring out our
income taxes), our bodies are prepared for just that.

PARASYMPATHETIC DIVISION

The other name for the parasympathetic division is
the craniosacral division. The cell bodies of parasym-
pathetic preganglionic neurons are in the brain stem
and the sacral segments of the spinal cord. Their axons
are in cranial nerve pairs 3, 7, 9, and 10 and in some
sacral nerves and extend to the parasympathetic gan-
glia. These ganglia are very close to or actually in the
visceral effector (see Fig. 8–12), and contain the post-
ganglionic cell bodies, with very short axons to the
cells of the effector.
In the parasympathetic division, one preganglionic
neuron synapses with just a few postganglionic neurons
to only one effector. With this anatomic arrangement,
very localized (one organ) responses are possible.

The parasympathetic division dominates in relaxed

(non-stress) situations to promote normal functioning

of several organ systems. Digestion will be efficient,

with increased secretions and peristalsis; defecation

and urination may occur; and the heart will beat at a

normal resting rate. Other functions of this division

are listed in Table 8–5.

Notice that when an organ receives both sympa-

thetic and parasympathetic impulses, the responses are

opposites. Such an arrangement makes maintaining an

appropriate level of activity quite simple, as in chang-

ing the heart rate to meet the needs of a situation.

Notice also that some visceral effectors receive only

sympathetic impulses. In such cases, the opposite

response is brought about by a decrease in sympathetic

impulses. Secretion by the sweat glands is an example.

NEUROTRANSMITTERS

Recall that neurotransmitters enable nerve impulses to

cross synapses. In autonomic pathways there are two

synapses: one between preganglionic and postgan-

glionic neurons, and the second between postgan-

glionic neurons and visceral effectors.

Acetylcholineis the transmitter released by all

preganglionic neurons, both sympathetic and para-

The Nervous System 189

Table 8–4 CRANIAL NERVES

Number and Name Function(s)

I Olfactory

II Optic

III Oculomotor

IV Trochlear

V Trigeminal

VI Abducens

VII Facial

VIII Acoustic (vestibulocochlear)

IX Glossopharyngeal

X Vagus

XI Accessory

XII Hypoglossal

• Sense of smell


• Sense of sight


• Movement of the eyeball; constriction of pupil in bright light or for near vision


• Movement of eyeball


• Sensation in face, scalp, and teeth; contraction of chewing muscles


• Movement of the eyeball


• Sense of taste; contraction of facial muscles; secretion of saliva


• Sense of hearing; sense of equilibrium


• Sense of taste; sensory for cardiac, respiratory, and blood pressure reflexes;
  contraction of pharynx; secretion of saliva


• Sensory in cardiac, respiratory, and blood pressure reflexes; sensory and motor
  to larynx (speaking); decreases heart rate; contraction of alimentary tube
  (peristalsis); increases digestive secretions


• Contraction of neck and shoulder muscles; motor to larynx (speaking)


• Movement of the tongue